1. Field of the Invention
This invention relates to abradable seals and seal materials and, more particularly, to an abradable seal for the stator casing of a fan or a compressor of a gas turbine engine for aircraft.
2. Description of the Prior Art
In the conventional, relatively cooler operating portion of a gas turbine engine, an organic lightweight abradable material secured to the casing of the stator is used to provide an abradable rub surface to the ends of fan or compressor blades. As shown in U.S. Pat. No. 3,547,455, issued Dec. 15, 1970 to J. E. Daunt; U.S. Pat. No. 3,843,278, issued Oct. 22, 1974 to S. P. Totell; and U.S. Pat. No. 4,460,185, issued Jul. 17, 1984 to M. F. Grandey, the abradable material may consist of an epoxy resin or a halogenated hydrocarbon including chopped fiberglass or phenolic microballoons or both.
Typically, a rub shroud of epoxy with phenolic microballoons is used as a filler and is structurally supported by a plastic honeycomb structure which is fixed to the inner annular surface of the casing of the stator to provide a structure on which to build a thick layer of the microballoon abradable material. The honeycomb may be made from a polyamide or aramid paper sold by Dupont Chemical Company under the trademark "Nomex", which is formed into a honeycomb that is dipped into a phenolic resin to stiffen the structure and is sold by Hexcel Corp. under the trademark "HRH 10". The honeycomb is bonded directly to the surface of a layer of fiberglass that is then bonded to the surface of the stator casing. The layer of fiberglass provides a textured surface for the microballoon abradable material to bond to and a thermal barrier between the metal (e.g., aluminum, steel, or titanium,) casing and the microballoon abradable material. The abradable material has a much higher coefficient of thermal expansion of the two materials which can cause cracking of the abradable material as a result of CTE mismatch exposed to variations in temperature. As a result, the abradable material is prone to falling out of the honeycomb cells.
During manufacture, the abradable material must be troweled into the honeycomb cells in an operator sensitive operation that often leaves voids and empty spots in the honeycomb. The application of the fiberglass, the honeycomb structure, and the abradable material each require a separate cure cycle which further complicates the manufacturing process.